The invention relates to co-gelling agents for viscoelastic surfactant fluid systems. More particularly it relates to selection and optimization of co-gelling agents for fluid systems to be used over broad ranges of salinity and temperature. Most particularly it relates to co-gelling agents to shorten shear recovery times and increase the viscosity of VES's for use in oilfield treatment fluids.
Certain surfactants, when in aqueous solution, form viscoelastic fluids. Such surfactants are termed “viscoelastic surfactants”, or “VES's”. Other components, such as additional VES's, co-surfactants, buffers, acids, solvents, and salts, are optional or necessary and, among other functions may increase the stability (especially thermal stability) or increase the viscosity of the systems by modifying and/or stabilizing the micelles; all the components together are called a viscoelastic surfactant fluid system. Not to be limited by theory, but many viscoelastic surfactant systems form long rod-like or worm-like micelles in aqueous solution. Entanglement of these micelle structures gives viscosity and elasticity to the fluid. For a fluid to have good viscosity and elasticity under given conditions, proper micelles must be formed and proper entanglement is needed. This requires the surfactant's structure to satisfy certain geometric requirements and requires the micelles to have sufficient length or interconnections for adequate entanglements.
Many chemical additives are known to improve the theological behavior (greater viscosity and/or greater stability and/or greater brine tolerance and/or lower shear sensitivity and/or faster rehealing if micelles are disrupted, for example by shear). Such materials are typically called co-surfactants, co-gelling agents, rheology modifiers, or rheology enhancers, etc., and typically are alcohols, organic acids such as carboxylic acids and sufonic acids, sulfonates, and others. We shall use the term co-gelling agents here. Such materials often have different effects, depending upon their exact composition and concentration, relative to the exact surfactant composition (for example hydrocarbon chain lengths of groups in the surfactant and co-surfactant) and concentration. For example, such materials may be beneficial at some concentrations and harmful (lower viscosity, reduced stability, greater shear sensitivity, longer rehealing times) at others.
In particular, there is a need for chemical additives that are effective for increasing the viscosity of VES systems at a given temperature, and/or for increasing the temperature at which such VES systems maintain the viscosities that make the fluids useful. Furthermore, many VES fluid systems exhibit long viscosity recovery times after experiencing prolonged high shear. Slow recovery after shear negatively impacts drag reduction and proppant transport capability, which consequently leads to undesirably high treating pressures and risks of near wellbore screen-outs. To overcome the deleterious effects of slow shear recovery, higher VES concentrations may sometimes be used. There is a need for additives that extend the conditions under which VES systems can be used, and reduce the amount of surfactant needed, which in turn reduces the cost and improves clean-up in many uses, such as, but not limited to, uses as oilfield treatment fluids, especially stimulation fluids, most especially hydraulic fracturing fluids. Although additives are known that can shorten VES shear recovery times and increase viscosities (see for example U.S. patent application Ser. Nos. 10/994,664 and 11/012,446, both of which are assigned to the same assignee as the present application and both of which are hereby incorporated in their entirety), there is still a need for additional simple, inexpensive rheology enhancers.